nvmet: return bool from nvmet_passthru_ctrl and nvmet_is_passthru_req

The target core code never needs the host-side nvme_ctrl structure.
Open code two uses of nvmet_is_passthru_req in passthru.c, and then
switch the helpers used by the core to return bool.  Also rename the
fuctions to better match their usage:

  nvmet_passthru_ctrl -> nvmet_is_passthru_subsys
  nvmet_req_passthru_ctrl -> nvmet_is_passthru_req

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>